1. Field of the Invention
This invention pertains to marine seismic sources and more particularly to an improved seismic source having a plurality of flexible, expansible enclosures, each enclosure utilizing an explosive gaseous mixture.
2. Description of the Prior Art
Various seismic wave generators have been employed in the past, starting with controlled dynamite charges as a source of energy. The most promising of the various sources employed today use explosive gas mixtures which explode either in open-bottomed domes or into various enclosures.
One very successful gas exploder employing a closed enclosure is disclosed in U.S. Pat. No. 3,480,101 entitled "Seismic Wave Source Using Explosive Gas in an Expansible Enclosure," issued Nov. 25, 1969 in the name of Adelbert Barry et al. The disclosure of this patent is incorporated by reference for all purposes.
A feature of the device disclosed in patent '101 and similar devices is that when the device is set off beneath the surface of the water, there is a primary energy shock imparted to the water and, after a period of time, there is an after-shock imparted to the water. The most popular explanation as to why and when this after-shock energy pulse occurs involves the effect of the hydrostatic pressure of water on the expansion enclosure or sleeve. That is, the sleeve expands the first time with gas explosion against the hydrostatic pressure of the water on the sleeve. This pressure causes the sleeve to contract or collapse; however, the gas in the chamber is sufficient, as there is pressure relief around the sleeve caused by the initial shock, to cause a second and lesser expansion to occur. Hence, there is an occurrence of a second shock.
Although the dynamics of gas expansion and external hydrostatic pressure and other related phenomena may not be completely understood or comprehended, the after-shock occurrence is undeniably present and very observable. Furthermore, it is also known that as the depth of the seismic source increases, the time between the primary shock and the after shock decreases, again probably because of the dynamic effect of the increased hydrostatic pressures at the greater depths.
A single primary source of the type described in the '101 patent produces a pulse with a relatively uniform energy amplitude over a range of frequencies from about 40 Hz to 1200 Hz. It is known that the energy in the higher frequencies of this range are not particularly useful for exploration targets deeper than about 3000 feet; therefore, it is desirable to expend proportionally more energy at lower frequencies than is available in known prior art sources.
A primary pulsing of energy from a seismic energy source of the type described above has both a positive portion and a negative portion immediately following as a result of reflection from the water surface. The after-shock pulsing of energy also includes both a positive and negative portion at a slightly later time, as mentioned above. The net effect of the two shocks of the type described occurring one after the other is that, in the frequency domain, one or more notches are established in the lower part of the frequency spectrum of interest. It is desirable to eliminate or greatly smooth the notching effect that results from operation of the source such as described above.
Sources which are activated on the surface of the water only have components of radiating energy outwardly and downwardly. A source that is activated somewhat below the surface of the water produces a wave in an upward direction, as well, which returns as a reflected or ghost pulse in a negative polarity with respect to the initial positive polarity. If there is no escaping energy at the surface at the time of reflection (i.e., no surface "blow-out"), then substantially the same amount of energy is reflected as initially produced. When there is "blow-out", then the amount of energy reflected is equal to one bar (i.e., one atmosphere of pressure). Having blow-out is desirable in a sense. A large shock produces a large positive portion, but a negative portion that does not exceed the predetermined value of one bar. Therefore, the result is a relatively large positive pulse and a relatively limited negative pulse, which is a desirable source shape.
However, it may be seen that a deeper source has to produce a greater shock to produce blow-out than a shallower source. This means that the positive and negative energy amplitude remain the same at a higher level for the deeper sources, resulting in notching and other undesirable characteristics at a higher energy level. It is desirable to have a limited negative portion with respect to the positive portion of an energy shock, even at greater depths, which has not been heretofore possible with the known prior art source devices.
Therefore, it is a feature of the present invention to provide an improved seismic marine source of the gas expansion type to enhance the primary shock energy therefrom while not enhancing the after-shock energy.
It is yet another feature of the present invention to provide an improved seismic marine source of the gas expansion type to smooth the frequency spectrum notches at the lower frequencies.
It is still another feature of the present invention to provide an improved seismic marine source of the gas expansion type that produces large positive energy pulses while producing relatively small negative energy pulses even from sources located relatively deeply in the water.
It is yet another feature of the present invention to provide an improved seismic marine source of the small diameter sleeve, gas-expansion type to enhance the output in the frequency range from about 25-250 Hz, which is an improvement over prior art sources.